Banks would have to hold enough easy-to-sell assets to survive a 30-day credit drought under a rule to be proposed today by the Federal Reserve that may have the greatest effect on banks with big trading operations such as JPMorgan Chase & Co. and Goldman Sachs Group Inc.

The Federal Reserve has expanded its scrutiny of banks’ physical commodities operations to encompass businesses run by Goldman Sachs Group Inc. and Morgan Stanley that Congress had previously authorized.